Variable matrix decoder for use in 4-2-4 matrix playback system

ABSTRACT

A decoder for use in a four channel playback system includes a control unit and a variable matrix. The control unit produces first and second control outputs which vary in opposite directions in accordance with the phase relationship between two channel signals. The variable matrix includes a first variable matrix circuit for producing two outputs related to the front channels and having matrix coefficients controlled by the first control output from the control unit and a second variable matrix circuit for producing two outputs related to the rear channels and having matrix coefficients controlled by the second control output from the control unit. The control outputs are used to improve the separation between front channels and to degrade the separation between rear channels, and vice versa, thereby enhancing the sense of presence of listeners in a reproduced sound field. The present invention discloses another decoder which comprises a first control unit for front-rear control and a second control unit for left-right control whereby front-left and right channels and rear-left and right channels are independently controlled.

Ito etal. i

[ VARIABLE MATRIX DECODER FOR USE IN 4-2-4 MATRIX PLAYBACK SYSTEM I R1111 3,825,684 1451 Jul 23', 1974 Primary Examiner Kathleen H. Claffy Assistant ExaminerThomas DAmico [75] Inventors: Ryosuke Ito; Susumu Takahashi, A f if Agent or FlrmHal-r.ls Kern &

- both of Tokyo, Japan ms ey t [73] Assignee; ansui Electric Co., Ltd., Tokyo, [57] ABSTRACT apan A decoder for use in a four channel playback system Flledi Q 1972 includes a control unit and a variable matrix. The con- [211 App] No 298 933 trol unit produces first and second control outputs which vary in oppositev directions in accordance with the phase relationship betweentwo channel signals. Foreign Appllcatlon l y D The variable matrix includes a first variable matrix cir- Oct. 25, 1971 Japan 46-84484 uit for p oducing two outputs related to the front Oct. 30, 1971 Japan 46-86521 L a nels and having matrix coefficients controlled by Nov. '4, 1971 Japan 46-87832 the first control output from the ontro unit and Nov. 10, 1971 Japan... 46-89678 s cond ar able matrix circuit for producing two out- Dec. 29, 1971- 'Japan .1 46-2145 putsrrelated to the realfl hanne s and .havingmatrix co- Man-23,1972 Japan 47-29332 efficients controlled by thesecond control output Apr. 17, 1972 Japan 47-38407 from e control nit. T he control outputs are used to improve the separation between front channels and to [52] US. Cl. 179/1 GQ, 179/ 100.4 ST, 179/100.1 TD degrade the separation between rear channels, and [51] Int. Cl H04r.5/00 ce e sa, the eby enhancing the sense of presence of [58] Field of Search179/l GQ, 1 GP, 1' G, 100.4 ST, 1 listeners in a reproduced sound field. The present in- I 179/100 1 TD, 15 BT vention discloses another decoder which comprises a first control unit for front-rear control and a second [56] 1 References Cited control unit for left-right control whereby front-left UNITED STATES PATENTS and right channels and rear-left and right channels are 3,786,193 1/1974 .Tsurushima .Q. 179/1 Go m controlled" v 7 34 Claims, 26 Drawing Figures v 27-3 PHASE -smFTER Ami 5 3 8 1 -J o:- 4O nae 0; 5 i FL EC2 J00 CONTROL V v UNIT 46 LAMP CONTROL ClRCUlT SHIFTER +J PATENltnJuLzsmm SHEET 03 0F 15 Now 5 mm mm mm; 3 I m 8 +m a H M a on 15 mm 6 I mo ziiuma M311 FIG. 6

PAIENTED Jmzaxsu min UGBHS' wasxsm PAIEN-IEBmzaan sumnaur 15- FIG.

PATENTEU JUL 2 31974 3, .584

swan-1n nr1s FIG. 248

VARIABLE MATRIX.- DECODER FoRjfUsE IN 14-2-4 MATRIX PLAYBACK SYSTEM" This invention relates to a directional sound system whereinat least four directional audio input signals are encoded into two channel signals andthe two channel signals are decoded into at least four audio signals corresponding to the directional audio input signals.

Recently; so-called 4-2-4 matrix playback systems have been used commercially. In such systems four channel directional audio input signals produced in'an original sound field are transformed by an encoder into two channel signals to be recorded on such recording media as stereo phonograph records, magnetic tapes and the like, and the two channel signals reproduced from the recording media are transformed by means of a decoder into four channel audio signals approximating said four directional audio input signals which are applied to four speakers disposed about listeners in the reproduced soundfield through suitable amplifiers. As above described, in this type of the 54-24 matrix playback system since the four directional audio input signals are transformed into two channel signals by the encoder it is impossible for the decoder to reproduce signals perfectly identical to 'the original four directional audio input signals. As a result, the cross-talk between adjacent channels in the reproduced sound signalincreases greatly so that it is-impossible to obtain a directional effect quite identical to that in theoriginal sound field and an optimum listening area in a listening room is restricted to a very small area.

It is an object of this invention to provide an 'improved .4-2-4matrix playback system decoder capable of greatly improving the separations between respective reproduced signals, between front and rear channels and between left and right channels in the forward or rearward.

Another object of this invention is to provide an improved decoder capable of eliminating cross-talks between respective channels thereby improving the qualityof the sound field and greatlywidening the listening area. I

According to this invention, there is provided .a decoder for use in a directional sound system wherein at least four directional audio input signals are encoded into two channel signals and the two channel signals are decoded into at least four audiosignals corresponding to the audio input signals, characterized in that the-decoder comprises a control unit responsive to the phase relationship between the two channel signals for producing first and second control signals,. first matrix means connected to receive the two channel signals for producing at least two audio output signals corresponding to at least two directional audio input signals, said matrix means having variable matrix coefficients which are controlled by the first control signals, and second matrix means connected to receive the two channel signals for producing at least two audio output signals corresponding to two remaining audio input signals, the second matrix means having variable matrix coefficients controlled by the second control signals.

The present invention can be more fully understood from the following detailed description when taken in connection with reference to theaccompanying drawings, inwhich:

FIG. 1 is a block diagram useful 'to explain the ple of the 4.- 2-4matrixsyste m;

princi- FIG. 2 shows a connection diagram of an encoder; FIG. 3 shows a block diagram of a decoder embodyingthe invention; v y r FIG. 4 shows a block diagram of a modified decoder;

FIG. 5 shows a circuitof a phase discriminator utilized as a control unit;

FIG. 6 is a diagram showing output characteristics of the phase discriminator shown in FIG. 5;

FIG. 7 is a plot showing a phase relationship between 2-channel signals; 7

FIG. 8 is a circuit diagram of a comparator utilized as a control unit;

FIG. 9 shows a circuit diagram of a modified decoder;

FIG. 10 shows a block diagram of another modification of the decoder;

' FIG. 11 is a circuit diagram of the modified decoder shown in FIG. 10;

' FIG. 12 shows a modification of a portion of the decoder, shown in FIG. 11 V FIG. 13 shows a block diagram of a modified decoder capable of independently controlling the signals in the front-left and right'channels and. in-the rear-left and right channels;

'FIG.14 shows the relationship between the phase angle between signals and the location of the sound source;

FIG. 15 is a graph showing the manner of varying the gains of various variable gain amplifiers utilized in the circuit shown in FIG. 13;

FIGS. 16 to 23 are diagrams showing the shifts of demodulation vector for a specified channel under various operating conditions of the decoder shown in FIG.

FIGS. 24A and 248 show a circuit diagram of the variable matrixcircuit shown in FIG. 13; and

FIG. 25 shows amodification of the decoder shown in FIG. 13.

To have a better understanding of the invention the principles of the 4-2-4 matrix playback system and an encoderwill firstbe described with reference to FIGS. 1 and 2 of the accompanying drawings.

- In' the system shown in FIG. 1, four microphones MFL, MFR, MRL and MRRare installed in an original sound field 1 in order to produce four channel directional audio signals FL (front-left), FR (front-right), RL (rear-left) and RR (rear-right).;T hese four channel signals are supplied to an encoder 2 to be transformed into two signals L and R. The outputs L and R from the encoder 2 are applied through two channel paths 3 and 4 to a decoder 5 to be transformed into reproduced four channel signals FL, FR, RR and RL' approximating original four channel signals FL, FR, RR and I channel stereo reproducing system.

There are many types of two channel systems which couple the outputs L and R from the encoder 2 to the decoder 5. According to one system the two outputs L and R from the encoder 2 are recorded on a recording medium such as a stereo phonographic record or a magnetic tape and the outputs from the recording medium are applied to the decoder S. According to the 3 a other system, the two outputs from the encoder 2 or the outputs reproduced from the recording medium are transmitted to the decoder 5 over an FM stereophonic broadcasting system.

The encoder 2 shown in FIG. 1 has a construction illustrated in FIG. 2. More particularly, directional audio signals FL and FR produced by the microphones MFL and MFR disposed at the front side of the original sound field 1 are supplied to a first resistive matrix circuit comprising serially connected resistors 11, 12 and 13. The directional audio signals RL and RR in the rear side of the original sound field l are applied to a second resistive matrix circuit 14 comprising serially connected resistors 15, 16 and 17. A signal from the upper terminal of the center resistor 12 of the first matrix circuit 10 and a signal derived out from the upper terminal of the center resistor 16 of the second matrix circuit 14 and phase shifted by a.:l-90 (+j) phase shifter 18 are combined by an adder 19 to produce a first channel or left signal L. A signal derived out from the lower terminal of the center resistor 12 of the first matrix circuit 10 and a signal derived out from'the lower terminal of the center resistor 16 of the second matrix circuit l4and phase shifted by a 90 (-j) phase shifter 20 are combined by an adder 21 to produce a second channel or right signal R. It is to be understood that phase shifters 18 and 20 are constructed to provide substantially the same phase shift over the entire audi ble frequency band.

Thus, the L and R signals are expressed as follows:-

L=FL+AFR+jRL+JARR R=FR +AFL jRR- jARL where A denotes a transformation constant or a matrix constant generally having a value of approximately 0.414. The reproduced four channel signals FL, FR, RL and RR are produced in the following manner by an ordinary decoder having the same fixed matrix constant A:

Let us' now consider the separation between respective channels in the reproduced sound field.

Let us assume that only the signal FL from the microphone MFL is present in the original sound field 1. Then, the reproduced four channel signals in the reproduced sound field will be expressed as follows:

Since A 0.414, the separations between channel FL and adjacent channels FR and RL are respectively equal to 3dB and the separation between the channels FL and RR in a diagonal direction equals dB as can be readily understood by those skilled in the art. As above described, since the separation between adjacent channels equals -3dB it is impossible to enjoy the ste- .reo playback'of four channels with a sufficiently large directional resolution.

' FIG. 3 shows a block diagram of an improved decoder embodying the invention including a variable matrix circuit having a matrix coefficient whose magnitude is controlled in accordance with the phase difference between two channel signals L and R.

In the decoder shown in FIG. 3, the two channel signals L and R are applied to the input terminals 23 and 24 of the decoder through two-channel media and hence to the input terminals 26-1 and 26-2 of the variable matrix circuit 25 which operates to decode or dematrix the two channel signals L and R to produce four channel signals on its output terminals 27-1, 27-2, 27-3 and 27-4. The output terminals 27-1 and 27-2 of the variable matrix circuit 25 are coupled to the output terminals 28-1 and 28-2 of the decoder, whereas output terminals 27-3 and 27-4 of the variable matrix circuit 25 are coupled to the output terminals 28-3 and 28-4 of the decoder respectively through phase shifters 29 and 30. Further, the variable matrix circuit 25 is provided with control'input terminals 31-1 and 31-2 to which are applied the control outputs ECl and EC2 of a control unit 32. The control unit 32 provides these control outputs ECl and EC2 in accordance with the phase difference between two-channel signals L and R. The magnitudes of the first and second control outputs ECl and EC2 from the control unit 32 vary in the opposite directions in proportion to the phase difference between signals L and R. The first control output EC 1 is used to control the matrix coefficient related to the front channels, whereas the second control output EC2 is used to control the matrix coefficient related to the rear channels. Where the phase difference between signals L and R is near zero, for instance, the first control output ECl operates to decrease the matrix coefficient related to the front channels thus enhancing the separation between front channels. On the other hand, the second control output EC2 operates to increase the matrix coefficient related to the rear channels thus reducing the separation between rear channels. Concurrently therewith the signal levels d the front channels are increased and those of the rear channels are decreased thus. improving the separation between the front and rear channels.

The control unit 32 may be constituted by a phase discriminator which detects directly the phase difference between signals L and R or a comparator which detects the phase relationship between signals L and R v in terms of the difference in the levels of a sum signal (L-l-R) and a difference signal (L-R). In this invention, the reason for controlling the matrix coefficient associated with the front and rear channels by detecting the phase relationship between signals L and R is as follows: Although a man has a keen ability to detecting the direction of a large sound but this sensitivity for a small sound coexisting with the large sound is very poor. For this reason, in the reproduction of four channel signals, where there is a large sound in the front side and a small sound in the rear side it would be possible to mroe efficiently enjoy the four channel playback by enhancing the separation between the front channels andrlowering the separation between the rear channels. On the contrary, where a small sound exists in the front side and a large sound in the rear side the four channel playback could be enjoyed more efficiently by enhancing the separation between the rear channelsiandlowering the separation between front channels.

it Where a large sound is present in the front and a small sound is present in the rear, that is, where FL,- FR RL, RR, signals L and R have substantially the same phase. This means that the level of a sum signal (L R) is higher than that of a difference signal (L Conversely, where alarge sound is present in the rear while a small sound is present in the front, that is, where FL, FR RL, RR, signals L and R have oppo site phase. In such a case, the level of the sum signal (L R) is lower than the level of the difference signal (L R). For-this reason, it. is possible to detect the phase relationship between signals L and R by'either a phase discriminator or a comparator. I

FIG. 4 is a connection diagram of one example of a decoder embodying the invention. The variable matrix circuit 25 .iS connected between input terminals-264 and 26-2 and comprisesa first resistive matrix circuit 34 including serially connected resistors 35, 36 and 37 42. The first matrix circuit 34 is associated with the front channels and its center resistor 36 comprises a photoconductiveelement such as a CdS element and the opposite terminals thereof are connected to output terminals 274 and 27-2. The second matrix circuit 38 is associated with the rear channels and its center resistor 40 also comprises a photoconductive element such as a CdSelement. The upper terminal of resistor 40 is connectedto an output terminal 27-3 while the'lower terminal to an output terminal 27-4 through an inverter 43. Incandescent lamps 44 and 45 for illuminating resistors 36 and 40, and lamp control circuits 46 and 47 for controlling the brightness of the lamps in accordance with the control outputs ECl and EC2 are connected to control input terminals 31-1 and 31-2 of the variable matrix circuit 25;

Before describing the operation of the decoder shown in FIG. 4, the construction of the control unit 32, animportant component of this invention, will be described hereunder. I FIG. 5 shows a circuit diagram of a phase discriminator which comprises a first limiter 50 including transistors 51 and 52 connected to receive the L signal and a second limiter 53 including transistors 54 and 55' con-- nected to receive the R signal; The first and second limiters S and 53 have large amplification gains and operateto transform the signals L and R into rectangular wave signals. Two output signals of opposite polarities produced by the second limiter 53 are amplified by first and second amplifiers 56 and 58 including transistors 57 and 59 respectively. The outputs from the first and second amplifiers 56 and 58 are supplied to a first switching circuit 60 and a second switching circuit 61. respectively includingbridge connecteddiodes D, to D and diodes D to D thereby causing these switching circuits ON and OFF alternately. The output from the 'age (in this case, +B/2 volts) through potentiometers 64 and 65, respectively. The slidable arms of the potentiorneters 64 arid65 supplies the first and second control outputs ECl and EC2.

The phase discriminator constructed as above described operates to switch the left signal L by alternately rendering ON and OFF the first and second switching circuits 60 and 61 in response to the right signal R thereby discriminatingthe phase difference between the right and left signals R and L. FIG. 6 shows the operating characteristic of the phase discriminator. showing that the first and'second control outputs ECl and EC2 vary symmetrically but in opposite directions about the rcfcrence level, which is equal to about +B/2 volts in the phase discriminator shown in FIG. 5. The case wherein the phase difference between the left and I right signals Land R equals zero degree corresponds to a case wherein the sound is present only in the front, that is L =fFL APR and R FR AFL. The case wherein the phase difference between the left and right signals L and R equals 180 corresponds to a case wherein the sound is present only inthe rear, that is L -+jRL jARR and R jRR jA RL, and the case wherein the phase difference between the signals L and R equals 90 corresponds to a case wherein sounds of the same level are present on the left hand and right hand sidesinthe front. as well as on the left hand and right hand sides in therear, that is L 1 j and R l j. FIG. 7 shows the relationship between the left signal L and the right signal R. Solid lines show the case wherein signals FL and FR alone are present and thus the signals L and R are in phase. Dotted lines show the 1 case wherein signals RL and RR alone are present and FIG. 8 shows a circuit diagram of a comparator I which is a modified embodiment of the control unit. In

this embodiment, the two channellsignals L and R are added and subtracted before they enter the comparator so as to form a sum signal (L R) and a difference signal (L R). The sum signal (L R) is applied to a logarithmic amplifier including a transistor 71 and diodes D9 and D whereas the difference signal (L R) is applied to a logarithmic amplifier 72 including a transistor 73 and diodes D and D The output from the amplifier 70 proportional to log (L R) is coupled to a boot strap circuit 74 comprising a transistor 75, the outputs thereof having opposite phases being applied across a, rectifier circuit 78 Similarly, the output from a the amplifier 72-which is proportional to log (L R) is supplied to a second boot strap circuit 76 including atransistor 77 and the outputs of the boot strap circuit 76 having opposite polarities are applied to a rectifier circuit 79. The output of rectifier 78 which is proportional to log I L+Rl and the output. of rectifier 79 which is proportional to log |L-R| are applied to the base electrodes of transistors 80 and 81 constituting a differential amplifier 82. A transistor 83 is connected to the emitter electrodes of transistors 80 and 81 to cause the amplifier 82 to act properly. The differential amplifier 82 acts to operate values log L+Rl/ L-Rl and log I L-R |/lL+Rl. The first control output ECl derived out from the collector electrode of transistor 80 corresponds to log lL+Rl l LRl whereas the second control output EC2 derived out from the collector electrode of 7 posite directions about the reference value in substantially the same manner as the outputs of the phase discriminator as shown in FIG. 6.

Returning back to FIG. 4, the operation of this decoderwill now be described. Where the phase difference between left and right signals L and R is about zero degree, that is where a large sound is present in the front and a small sound in the rear, the first control output ECl from the control unit 32 is large, whereas the second control output EC2 is small. As a result, the lamp control circuits 46 and 47 operate to pass a large current through the lamp 45 but a small current through the lamp 44 with the result that photosensitive element 40 manifests a small resistance value whereas the photosensitive element 36 a large resistance value. Accordingly, the level of the left signal L contained in the decoder output FL is increased but the level f the right signal R contributing to the cross-talkis decreased. Onthe other hand, the level of the right signal R contained in the output FR is increased whereas the level of the left signal L contributing to the cross-talk is decreased. This means an improvement of the separation between the front channels. The level of the left signal L contained in the decoder output RL associated with the rear channels is decreased whereas the level of the right signal R contributing to the-crosstalk is increased. On the other hand, the level of the right signal R contained in the output RR is decreased while the level of the left signal L contributing to the crosstalk is increased. This means a degradation of the separation between rear channels. Concurrently therewith, as the level of the signals of the front channels is increased and the level of the signals of the rear channels decreases, the separation between front and rear channels will also be improved. I

Where the phase difference between the left and right signals L and R is equal to approximately 180, the separation between the rear channels will be improved and that between the front channels will be degraded which is just opposite to those described above.

Where the phase difference between the left and right signals L and R is nearly equal to 90, that is wheresounds of thesame level presentin the forward as well as in the rearward the control output's'ECl and EC2 of the control unit will have the same level, it may be considered that the variable matrix circuit operates in the same manner as an ordinary fixed matrix circuit.

to another embodiment of the invention wherein a first matrix circuit 90 associated with the front channels comprises a first differential amplifier 91 including transistors 92 and 93. The left signal Lis coupled to the base electrode of transistor 92 while the right signal R 'is coupled to the base electrode of transistor 93 through an inverter 94 including a transistor 95. The collector electrode of transistor 92 is connected to the first output terminal 27-1 of the matrix circuit while the collector electrode of transistor 93 is connected to the second output terminal 27-2 of the matrix circuit through an inverter 96 comprising a transistor 97. A first control circuit 99 including a field effect transistor 100 is capacitively connected in parallel with a common emitter resistor 98 of transistors 92 and 93 which constitute the differential amplifier 91. The gate elec' trode of the field effect transistor 100 is connected to a control input terminal 31-1 so that it acts as a variable resistor. The first control circuit 99 operates to vary the AC impedance of the emitter circuits of transistors 92 and 93 in accordance with the magnitude of the control input ECl so 'as to control the gain of the differential amplifier 91. 1

The second matrix circuit associated with the rear channels comprises a second differential amplifier 106 including transistors 107 and 108. The left signal L is coupled to the base electrode of transistor 107, whereas the rightsignal'R is coupled to the base electrode of transistor 108. The collector electrodes of transistors 107 and 108 are connected to the third and fourth output terminals 27-3 and 27-4, respectively, of the matrix circuit. A second control circuit 110 including a field-effect transistor 111 is capacitively connected in parallel with a common emitter resistor 109 for transistors 107 and 108. The gate electrode of fieldeffect transistor 1 1 1 is connected to a control input terminal 31-2. The second control circuit 110 operates in thesame manner as the first control circuit 99 so as to control the gain of the second differential amplifier in accordance with the magnitude ofthe control input EC2.

The operation of the variable matrix circuit shown in FIG. 9 will be briefly described as follows: Where the left and right signals L and R are substantially in phase, the control input ECl is large and the control input EC2 is small. Consequently, the AC impedance of the emitter circuits of transistors 92 and 93 is decreased whereby the gain of the first differential amplifier 90 is increased, whereas that 'of the second differential'amplifier 106'is decreased. Increase in the. gain of the first differential amplifier 91 results in the increase in the level of the left signal L which is derived out from the collectorelectrode of transistor 92 and in the decrease in the level of the right signal R contributing to increasing the cross-talk. On the other hand, the level of the right signal R derived out from the collector electrode of transistor 93 is increased and the level of the left signal L contributing to increasing the cross-talk is decreased. Accordingly, the separation between the front channels is improved with the increase in the signal level. In the rear channels, as the gain of the second differential amplifier 106 decreases, the separation degrades with the decrease in the signal level.

The reproduced four channel outputs FL, FR, RL and RR'of this modified decoder can also be expressed by substantially the same equations used in the first embodiment of the decoder shown in FIG. 4.

FIG. 10 is a block diagram of a decoder according to another embodiment of the invention. With reference first to the front channels, there are provided a first matrix circuit adapted to produce sum signals (L R) and (L+R) of opposite polarities, and a second matrix 

1. A decoder for use in a directional sound system wherein at least four directional audio input signals are encoded into two channel signals and the two channel signals are decoded into at least four audio output signals corresponding to said audio input signals, said two channel signals having an amplitude ratio and a phase relationship, said decoder comprising: at least one control unit responsive to the phase relationship between said two channel signals for producing first and second control signals; first matrix means connected to receive said two channel signals for combining said two channel signals to produce two audio output signals corresponding to two directional audio input signals, said first matrix meAns including means responsive to said first control signal for varying at least the amplitude ratio of said two channel signals contained in each of said output signals; and second matrix means connected to receive said two channel signals for combining said two channel signals to produce two audio output signals corresponding to two remaining directional audio input signals, said second matrix means including means responsive to said second control signal for varying at least the amplitude ratio of said two channel signals contained in each of said output signals.
 2. A decoder according to claim 1 wherein each of said first and second matrix means comprises a resistive matrix circuit including at least one variable resistor, said two audio output signals are derived out from the opposite terminals of said variable resistor and the variable resistors of said first and second matrix means are controlled by said first and second control signals to vary the amplitude ratios of said two channel signals contained in said output signals.
 3. A decoder according to claim 2 wherein said variable resistors included in said first and second matrix means comprise photoconductive elements and wherein there are provided first and second light sources for illuminating said photoconductive elements and first and second control circuits responsive to said first and second control signals for controlling the brightness of said first and second light sources.
 4. A decoder according to claim 1 wherein said first matrix means comprises a first differential amplifier having first and second input terminals, said first input terminal being connected to receive one channel signal; means for reversing the phase of the other channel signal; means for coupling the output from said phase reversing means to the second input terminal of said first differential amplifier; and means responsive to said first control signal for controlling the gain of said first differential amplifier, and wherein said second matrix means comprises a second differential amplifier having first and second input terminals connected to receive said two channel signals respectively and means responsive to said second control signal for controlling the gain of said second differential amplifier.
 5. A decoder according to claim 1 further comprising: first means for producing a sum output of said two channel signals; second means for producing a sum output of said two channel signals having the opposite polarity to that of the output of said first means; third means for producing a difference output of said two channel signals; and fourth means for producing a difference output of said two channel signals having the opposite polarity to that of said third means; wherein said first matrix means includes first variable gain amplifier means connected to receive the output of said third means, said variable gain amplifier means being controlled in its gain in response to said first control signal; fifth means for combining the output from said first variable gain amplifier means and the output from said first means; and sixth means for combining the output from said first variable gain amplifier means and the output from said second means; and wherein said second matrix means includes second variable gain amplifier means connected to receive the output from said first means, said variable gain amplifier means being controlled in its gain in response to said second control signal; seventh means for combining the output from said second variable gain amplifier means and the output from said third means; and eighth means for combining the output from said second variable gain amplifier means and the output from said fourth means.
 6. A decoder according to claim 5 wherein said two channel signals are expressed by L FL + Delta FR + jRL + j Delta RR and R FR + Delta FL - jRR - j Delta RL where FL, FR, RL and RR represent directivE audio input signals and Delta represents a constant equal to about 0.414; the gain of said first variable gain amplifier means varies between substantially 0 and 2.41 in response to said first control signal; and wherein the gain of said second variable gain amplifier means varies between substantially 0 and 2.41 in the opposite direction to said gain of said first variable gain amplifier means in response to said second control signal.
 7. A decoder according to claim 1 wherein said control unit includes a phase discriminator for detecting the phase difference between said two channel signals.
 8. A decoder according to claim 7 wherein said phase discriminator comprises first means connected to receive one channel signal for producing a rectangular wave output; second means connected to receive the other channel signal for producing a rectangular wave output; third and fourth means which are connected to receive the output from said first means for producing outputs of opposite polarities; and first and second switching means which are alternately rendered ON and OFF in response to the outputs from said third and fourth means, the input sides of said first and second switching means being connected to receive the output of said second means to thereby produce first and second control signals from the output sides of said first and second switching means, the magnitudes of said first and second control signals varying in opposite directions in accordance with the phase difference between said two channel signals.
 9. A decoder according to claim 1 wherein said control unit includes a comparator for detecting the phase relationship between said two channel signals in accordance with level difference between sum and difference signals of said two channel signals.
 10. A decoder as claimed in claim 9 wherein said comparator comprises a first logarithmic amplifier connected to receive a sum signal of said two channel signals; a second logarithmic amplifier connected to receive a difference signal of said two channel signals; first means to rectify the output of said first logarithmic amplifier; second means for rectifying the output of said second logarithmic amplifier; and a differential amplifier connected to receive the outputs from said first and second means for producing first and second outputs, the magnitudes of which vary in the opposite directions in accordance with the phase relationship between said two channel signals.
 11. A decoder according to claim 5 wherein each of said first and second variable gain amplifier means has a frequency characteristic such that it manifests a relatively low gain for signals having frequencies less than a first predetermined frequency and a relatively high gain for signals having frequencies less than a second predetermined frequency regardless of the magnitudes of said first and second control signals.
 12. A decoder according to claim 11 wherein said first predetermined frequency is about 200 Hz and said second predetermined frequency is about 5000 Hz.
 13. A decoder according to claim 5 wherein each of said first and second variable gain amplifier means includes means for substantially preventing signals contained in the input signal and having frequencies lower than a first predetermined frequency from being applied to said amplifier means and means for operating said amplifier means at a high gain for signals contained in the input signal and having frequencies higher than a second predetermined frequency which is higher than said first predetermined frequency regardless of the magnitudes of said first and second control signals.
 14. A decoder according to claim 5 wherein each of said first and second variable gain amplifier means comprises a variable gain amplifier connected to mainly receive input signals having intermediate frequencies in the entire audible frequency band and a fixed gain amplifier connected to mainly receive input signals having higher freQuencies than said intermediate frequencies.
 15. A decoder according to claim 14 which further comprises a bandpass filter connected to the input of said variable gain amplifier and a high-pass filter connected to the input of said fixed gain amplifier.
 16. A decoder according to claim 5 further comprising a second control unit for producing third and fourth control signals in response to said two channel signals; third variable gain amplifier means connected to receive a first channel signal, said variable gain amplifier means being controlled in its gain by said third control signal; ninth means for producing a sum output of the output of said third variable gain amplifier means and a second channel signal; 10th means for producing a difference output of the output said third variable gain amplifier and the second channel signal; fourth variable gain amplifier means connected to receive the second channel signal, said fourth variable gain amplifier means being controlled in its gain by said fourth control signal; 11th means for producing a sum output of the output of said fourth variable gain amplifier end the first channel signal; 12th means for producing a difference output of the output of said fourth variable gain amplifier and the first channel signal; 13th means for combining the output of said fifth means and the output of said ninth means at a predetermined amplitude ratio; 14th means for combining the output of said sixth means and the output of said eleventh means with a predetermined amplitude ratio; 15th means for combining the output of said seventh means and the output of said tenth means at a predetermined amplitude ratio; and 16th means for combining the output of said eighth means and the output of said twelfth means with a predetermined amplitude ratio.
 17. A decoder according to claim 16 further comprising first and second phase shifters for introducing a predetermined phase difference between said two channel signals; adder means for adding the outputs of said first and second phase shifters; and subtractor means for producing a difference output of the outputs of said first and second phase shifters; and wherein said second control unit includes a phase discriminator for detecting the phase difference between the outputs of said adder means and subtractor means.
 18. A decoder according to claim 16 wherein said second control unit includes a comparator for detecting level difference between said first and second channel signals.
 19. A decoder according to claim 1 wherein said control unit produces said first control signal of the maximum level and said second control signal of the minimum level when the phase difference between said two channel signals is about 0*; said first and second control signals having the identical level when the phase difference is about 90*, and said first control signal of the minimum level and second control signal of the maximum level when the phase difference is about 180*.
 20. A decoder adapted to decode two channel signals L and R each containing at least three of four directional audio signals FR, FL, RL and RR, into reproduced four channel signals FR'', FL'', RL'' and RR'' respectively corresponding to said directional audio signals FR, FL, RL and RR, said decoder comprising: a control unit responsive to the phase relationship between said two channel signals L and R for producing first and second control outputs which vary in opposite directions with reference to a predetermined reference level; first matrix means connected to receive said two channel signals L and R for producing said reproduced signals FL'' and FR'' which are expressed by mL + nR and nL + mR, respectively, where m and n represent variable matrix coefficients which vary in the opposite directions in accordance with said first control output; and second matrix means connected to receiVe said two channel signals L and R for producing reproduced signals RL'' and RR'' which are expressed by pL - qR and qL - pR, respectively, where p and q represent variable matrix coefficients which vary in the opposite directions in accordance with said second control output and said coefficients m and p vary in the opposite directions by said first and second control outputs.
 21. A decoder adapted to decode two channel signals L and R, each containing at least three of four directional audio signals FR, FL, RL and RR into reproduced four channel signals FR'', FL'', RL'' and RR'' respectively corresponding to said directional audio signals FR, FL, RL and RR, said decoder comprising: a control unit responsive to the phase relationship between said two channel signals L and R for producing first and second control outputs which vary in the opposite directions with reference to a predetermined reference level; first matrix means connected to receive said two channel signals L and R for forming said reproduced signals FL'' and FR'' which are expressed by (1+f)L + (1-f)R and (1+f)R + (1-f)L, respectively, where f represents a variable matrix coefficient which is controlled by said first control output; second matrix means connected to receive said two channel signals L and R for forming said reproduced signals RL'' and RR'' which are expressed by (1+b)L - (1-b) R and (1+b)R - (1-b)L, respectively, where b represents a variable matrix coefficient which is controlled by said second control output, said coefficients b and f varying in the opposite directions.
 22. A decoder according to claim 21 wherein said two channel signals L and R are expressed respectively by FL + Delta FR + jRL + j Delta RR and FR + Delta FL - jRR - j Delta RL where Delta equals substantially 0.414 and wherein said variable matrix coefficients f and b vary in opposite directions between substantially 0 and 2.41 in response to said first and second control outputs.
 23. A decoder adapted to decode two channel signals L and R, each containing at least three of four directional audio input signals FL and FR associated with front left and right channels, RL and RR associated with rear-left and right channels, into reproduced four channel output signals, said decoder comprising: a first control unit responsive to the level ratio between the front and rear audio input signals contained in the two channel signals for producing first and second control signals, the magnitudes of which vary in opposite directions; a second control unit responsive to the level ratio between the left and right audio input signals contained in the two channel signals for producing third and fourth control signals, the magnitudes of which vary in opposite directions; first means connected to receive said two channel signals L and R for producing an output proportional to a signal (1+f+ Square Root 2)L + (1-f+ Square Root 2l)R, where f and l represent coefficients which vary over a predetermined range; second means connected to receive said two channel signals L and R for producing an output proportional to a signal (1+ f+ Square Root 2)R + (1- f+ Square Root 2r)L, where f and r represent coefficient which vary over a predetermined range; third means connected to receive said two channel signals L and R for producing an output proportional to a signal (1+ b+ Square Root 2)L - (1- b+ Square Root 2l)R, where b and l represent coefficients which vary over a predetermined range; fourth means connected to receive said two channel signals L and R foR producing an output proportional to a signal (1+b+ Square Root 2)R - (1-b+ Square Root 2r)L, where b and r represent coefficients which vary over a predetermined range; fifth means responsive to said first control signal for varying said coefficient f; sixth means responsive to said second control signal for varying said coefficient b; seventh means responsive to said third control signal for varying said coefficient r; and eigth means responsive to said fourth control signal for varying said coefficient l.
 24. A decoder according to claim 23 wherein said two channel signals L and R are expressed respectively by FL + Delta FR + jRL + j Delta RR and FR + Delta FL - jRR - j Delta RL where Delta equals substantially 0.414 and said coefficients f, b, r and l vary respectively between substantially 0 and 3.414.
 25. A decoder adapted to decode first and second channel signals, each containing at least three of four audio signals associated with front-left and right channels and rear-left and right channels, into four channel signals, said decoder comprising: first means responsive to the level ratio between the front and rear audio input signals contained in the two channel signals for producing first and second control signals; the magnitudes of which vary in opposite directions; second means responsive to the level ratio between the left and right audio input signals contained in the two channel signals for producing third and fourth control signals the magnitudes of which vary in opposite directions; third means for producing a difference signal of the first and second channel signals; fourth means responsive to said first control signal for varying the amplitude of the output signal from said third means; fifth means for producing a sum signal of the first and second channel signals; sixth means responsive to said second control signal for varying the amplitude of the output signal from said fifth means; seventh means responsive to said third control signal for varying the amplitude of the first channel signal; eighth means responsive to said fourth control signal for varying the amplitude of the second channel signal; ninth means for combining the first and second channel signals, the output signal from said fourth means, and the output signal from said eighth means with the same polarities and a first predetermined relative amplitude ratio; 10th means for combining the first and second channel signals, the output signal from said fourth means, and the output from said seventh means with a relationship wherein the output signal from said fourth means is opposite in polarity to the remaining three signals, and a second predetermined relative amplitude ratio; eleventh means for combining the first and second channel signals, the output signal from said sixth means, and the output signal from said eighth means with a relationship wherein the first channel signal and the output signal from said sixth means are opposite in polarity to the remaining two signals, and a third predetermined relative amplitude ratio; and 12th means for combining the first and second channel signals, the output signal from said sixth means and the output signal from said seventh means with a relationship wherein the second channel signal and the output signal from said sixth means are opposite in polarity to the remaining two signals, and a fourth predetermined relative amplitude ratio.
 26. A decoder according to claim 25 wherein said first amplitude ratio is substantially (1+ Square Root 2) : 1 : 1 : Square Root 2, said second amplitude ratio substantially 1 : (1+ Square Root 2) : 1 : Square Root 2, said third amplitude ratio substantially (1+ Square Root 2) : 1: 1 : Square Root 2, and said fourth amplitude ratio substantially 1 : 1 (1+ Square Root 2) : 1 : Square Root
 2. 27. A decoder according to claim 25 wherein said fourth and sixth means vary in opposite directions the amplitudes of the input signals therefor over a range between substantially 0 and 3.414, and said seventh and eighth means vary in opposite directions the amplitudes of the input signals therefor over a range between substantially 0 and 3.414.
 28. A decoder adapted to decode first and second channel signals, each containing at least three of four audio signals associated with front-left and right channels and rear-left and right channels, into four channel signals, said decoder comprising: first means responsive to phase relationship between the front and rear channels for producing first and second control signals, the magnitudes of which vary in opposite directions; second means for producing a difference signal of the first and second channel signals; third means responsive to said first control signal for varying the amplitude of the output signal from said second means; fourth means for producing a sum signal of the first and second channel signals; fifth means responsive to said second control signal for varying the amplitude of the output signal from said fourth means; sixth means for combining the first and second channel signals, and the output signal from said third means with the same polarities; seventh means for combining the first and second channel signals, and the output signal from said third means with a relationship wherein the first and second channel signals are opposite in polarity to the output signal from said third means; eighth means for combining the first and second channel signals and the output signal from said fifth means with a relationship wherein the second channel signal is opposite in polarity to the remaining two signals; and ninth means for combining the first and second channel signals and the output signal from said fifth means with a relationship wherein the first channel signal is opposite in polarity to the remaining two signals.
 29. A decoder according to claim 28 wherein said third and fifth means vary in opposite directions the amplitudes of the input signals therefor over a range from about 0 to 2.414.
 30. A decoder according to claim 16 wherein each of said variable gain amplifier means has a frequency characteristic such that is manifests a relatively low gain for signals having frequencies less than a first predetermined frequency and a relatively high gain for signals have frequencies less than a second predetermined frequency regardless of the magnitudes of said first and second control signals.
 31. A decoder according to claim 30 wherein said first predetermined frequency is about 200 Hz and said second predetermined frequency is about 5000 Hz.
 32. A decoder according to claim 16 wherein each of said variable gain amplifier means includes means for substantially preventing signals contained in the input signal and having frequencies lower than a first predetermined frequency from being applied to said amplifier means and means for operating said amplifier means at a high gain for signals contained in the input signal and having frequencies higher than a second predetermined frequency which is higher than said first predetermined frequency regardless of the magnitudes of said first and second control signals.
 33. A decoder according to claim 16 wherein each of said variable gain amplifier means comprises a variable gain amplifier connected to mainly receive input signals having intermediate frequencies in the entire audible frequency band and a fixed gain amplifier connected to mainly receive input signals having higher frequencies than said intermediate frequencies.
 34. A decoder according to claim 33 which further comprises a bandpass filter connected to the input of said variable gain amplifier and a high-pass filter connected to the input of said fixed gain amplifier. 